In the past, yarns generally were classified as being either staple fiber yarns or continuous filament yarns. Staple fibers are typified by most of the natural occurring fibers, such for example as cotton, wool, flax and the like. As various man made fibers were introduced, they were chopped into staple form in order to be compatible with the existing processes of opening, carding, drawing and spinning.
Continuous filament yarns usually are formed from synthetic filamentary material or from a naturally occurring filamentary material such, for example, as silk. In order to prepare a continuous filament yarn, a number of filaments are twisted together after extrusion for coherence.
For many years staple fiber yarns and continuous filament yarns have been distinct and separate. Recently, several yarn systems have been developed wherein continuous filamentary material and staple fibers are assembled to the same structure. Examples of such systems are core spinning, wrap spinning, self twist spinning and the like. All of these processes have two common features. First, each of the components, both filamentary and staple fiber material, retains its original form. Secondly, the resultant yarn is heterogeneous.
In many instances blended staple fiber yarns are desirable in that intimate mixtures may be achieved which permit exploitation of the relative merits of the component fibers. One example of such would be a blended polyestercotton yarn which combines the easy care characteristic of polyester with the comfort of cotton. In a blended staple fiber yarn the constituent fibers ideally should be similar in length and in initial modulus or stiffness measured in grams per denier. Where the staple fibers are combined, compatability and length ensures that the proportion of relatively short fibers is minimized for satisfactory processing during the spinning operation. This characteristic ultimately determines the evenness of the yarn and the range of spinning specifications which may successfully be used. Similarity of initial moduli is necessary to ensure that each component fiber makes it proportionate contribution to tensile properties, at least over the normal range of loading in ultimate usage. For these reasons, polyester staple used for blending with cotton generally is an inch-and-a-quarter to an inch-and-a-half in length with relatively high initial modulus whereas staple polyester used for blending with wool is two inches or more in length with a lower initial modulus. Occasionally in composite yarns the ideal of constant fiber modulus is not possible. For example, blends of wool and cotton or nylon and cotton are produced in yarn form. Rarely, however, are fibers of grossly dissimilar length combined.
There exist continuous filamentary materials which, for reason of ultimate use, require blending with other fibers in order to utilize the attributes of staple fiber yarns, particularly those of bulk and cover. In general, such continuous filamentary materials are weak, friable or of high initial modulus and would not withstand the loads imposed during fiber preparation and yarn production. One example of such a continuous filamentary material is filamentary activated carbon.
Activated carbon has long been recognized as an extremely useful material for many purposes owing to its high specific surface area and resultant adsorbent properties. While, strictly speaking, adsorption implies interaction at an outer surface, and absorption refers to introduction into the interstices of a substrate, the mechanism by which activated carbon incorporates foreign substances is such that adsorption occurs both at the outer surfaces as well as those surfaces which are physically continuous with the outer surfaces and yet extend into the body of the activated carbon. Consequently, in this instance the mechanisms of adsorption and absorption become interchangeable. For this reason we have hereafter referred to the incorporation of gases and liquids by activated carbon as adsorption.
Owing to its high degree of adsorbancy, activated carbon is used extensively as an adsorber in air purification, water treatment, chemical filtration, as well as in protective clothing and in filters in the nuclear industry. Since the activated carbon is typically used in the form of granules, powder, or microspheres, difficulty has always arisen in assembling the material into structures which take full advantage of the adsorptive capability of the material. Often, the particulate carbon or its aggregates are entrapped within rigid retaining structures with additional membranes to prevent shifting, sifting or settling, as well as to prevent physical removal of the carbon by the filtration process itself.
In addition to the mechanical methods of holding the carbon material described hereinabove, it is often physically or chemically bonded onto or into a supporting structure such as foam, fabric, paper and the like. By whatever means such bonding is effected, it invariably results in a chemical contamination or occlusion of the carbon which decreases its adsorptivity. Moreover, the bond between the carbon and its supporting substrate is subject to deterioration which often results in loss of carbon.
More recently, activated carbon has been produced in the form of filamentary fibrous activated carbon. This form of the material has the potential of being incorporated within structures owing to its significant length to diameter ratio as compared to that of the particulate form of activated carbon. Heretofore the potential of activated carbon fiber has not been fully realized owing to its inherent friability and resultant loss of material in processing.